The centrifugal force, given by the expression mw^2r, where m=mass, w=angular acceleration (radial component) and r=radius, is actually radially directed towards the centre of rotation, but is experienced by a reactive force acting outwards, because of the pressure of the seat against the passenger's body. At the apex of the rotation, gravity is pulling the passenger vertically downwards with a force mg, where g=9.81m/s/s, the acceleration of gravity, so the net effect is m(w^2r-g)=696 N. Therefore, 82(15w^2-9.81)=696. From this w^2=(696/82+9.81)/15=1.22 approx. At the lowest point the force is m(w^2r+g)=82(1.22*15+9.81)=2305 N approx.
We need to convert this force into a weight by dividing by 9.81: 2305/9.81=235kg approx. (2.9g).